With the removal of the NAT64/46 modules, we can now simplify the
internals of our `IpPacket` struct. The requirements for our `IpPacket`
struct are somewhat delicate.
On the one hand, we don't want to be overly restrictive in our parsing /
validation code because there is a lot of broken software out there that
doesn't necessarily follow RFCs. Hence, we want to be as lenient as
possible in what we accept.
On the other hand, we do need to verify certain aspects of the packet,
like the payload lengths. At the moment, we are somewhat too lenient
there which causes errors on the Gateway where we have to NAT or
otherwise manipulate the packets. See #9567 or #9552 for example.
To fix this, we make the parsing in the `IpPacket` constructor more
restrictive. If it is a UDP, TCP or ICMP packet, we attempt to fully
parse its headers and validate the payload lengths.
This parsing allows us to then rely on the integrity of the packet as
part of the implementation. This does create several code paths that can
in theory panic but in practice, should be impossible to hit. To ensure
that this does in fact not happen, we also tackle an issue that is long
overdue: Fuzzing.
Resolves: #6667Resolves: #9567Resolves: #9552
